The present invention relates to an apparatus and method for recording and/or reproducing information on an optical information recording medium based on holography.
In recent years, an optical disc having a recording density of about 50 GB based on blu-ray disc specifications using a blue-violet semiconductor laser has reached commercial fruition even as a consumer-oriented product. In the future, it is desirable even for such an optical disc to have a large capacity of 100 GB to 1 TB nearly the same as a HDD (Hard Disk Drive).
In order to attain such an ultra-high density with use of an optical disc, however, it becomes necessary to use a higher-density technique based on a new scheme different from a higher-density technique using a shorter wavelength and using an objective lens having a higher NA (numerical aperture).
Among studies relating to next-generation storage techniques, much attention is focused on a holography technique for recording digital information based on holography.
A hologram recording technique refers to a technique for recording information on a recording medium by overlapping signal light having information on page data two-dimensionally modulated by a spatial light modulator with a reference light within the recording medium and by causing refractive index modulation within the recording medium by then-generated interference fringe pattern.
In an information reproduction mode, irradiation of the reference light used in a recording mode on the recording medium causes a holography already recorded in the recording medium to act like a diffraction grating and to produce diffraction light. The diffraction light is reproduced as the same light including the recorded signal light and phase information.
The reproduced signal light is detected two-dimensionally at a high speed with use of such an optical detector as a CMOS or a CCD. In this way, by the hologram recording technique, two-dimensional information can be collectively recorded on an optical recording medium with use of a single hologram, the recorded information can be reproduced, and a plurality of pieces of page data can be overwritten at a given location on the recording medium. As a result, a large capacity of information can be recorded and reproduced at a high speed on the recording medium.
One of the holography recording techniques is disclosed, for example, in JP-A-2004-272268. This publication describes a technique for recording information by multiplexing holograms.